Eosinophils are resident immune cells of the healthy gastrointestinal (GI) tract. Once considered exclusively inflammatory cells that support allergic (Th2) inflammation, recent evidence demonstrates a role for eosinophils in supporting a more broad set of inflammatory responses and in the maintenance of metabolic homeostasis. Indeed, eosinophils are a component of both active and inactive lesions in inflammatory bowel disease (IBD). Although eosinophils become activated upon entry into tissues, the factors controlling their homeostasis and the consequences of eosinophil activation in the GI tract remain enigmatic. The GI tract and associated lymphoid tissues (GALT) are in apposition with commensal flora, food protein and environmental antigen. These factors act on multiple innate immune cell populations in the GI tissue including antigen presenting cells, which prime adaptive immune responses, and resident accessory cells which shape the outcome of these responses at tissue sites. Eosinophils are a prominent population of resident cells within lamina propria of the small and large intestine. However, the function of these resident cells and how they are regulated by the dominant signals present in the gut: the commensal microbiota and the dietary metabolites, is unclear. Our preliminary data suggest that commensal microbiota and the essential dietary nutrient, Vitamin A, strongly influence eosinophil activation and survival respectively. Sensing of dietary and microbial cues in the gut by resident innate cells is key to priming and sustaining CD4+ T effector responses (Teff) secreting IFN-3 (Th1) and/or IL-17 (Th17). Accordingly, our preliminary data using the eosinophil deficient dblGATA mouse model, demonstrate that the absence of eosinophils results in impaired ability to mount Teff responses in the gut. The overarching goal of this application is to 1) define the role of commensal bacteria and dietary components in controlling GI eosinophil function and homeostasis and 2) define the mechanisms by which eosinophils are regulating effector immune responses in the GI tract. Better understanding GI eosinophils will help expand knowledge of their pathogenic role in IBD. These studies will elucidate novel pathways of eosinophils in contributing to Teff responses in the GI tract and may provide novel therapeutic targets to both prevent pathology in IBD, but also to promote beneficial immunity in the context of mucosal vaccines.